This invention relates generally to display systems for digital oscilloscopes, and in particular to a system for reproducing the envelope of complex signals such as carrier signals.
Real-time oscilloscopes provide a continuous time-based display of the instantaneous amplitude values of electrical phenomena, and are thus able to accurately display the waveforms of complex signals, such as high-frequency carrier signals having low-frequency envelopes. These types of waveforms, as well as other types, are also displayable by real-time oscilloscopes equipped with direct-view bistable storage tubes because the signal processing circuits and recording medium are continuous. On the other hand, digital oscilloscopes chop input signals into time points determined by an internal clock, quantize the instantaneous amplitude values at those points, and store the resulting digital representations in digital memory. The display is regenerated from the memory at a predetermined clock rate, and is manifested either as a series of dots, or connected dots. Since the input signals are not functionally related to the internal clock of the digital oscilloscope, whatever the instantaneous value of the input signal happens to be when the clock edge occurs is what gets stored. The information between such points, of course, is lost, so that for complex signals, an intelligible waveform is difficult, if not impossible, to reconstruct.
In order to observe such complex signals, an envelope display system has been developed for digital oscilloscopes and is disclosed in U.S. Pat. No. 4,251,815 which is assigned to the assignee of the present invention. According to this system, the maximum and minimum amplitude values of repetitive waveform are determined by a max-min detector circuit, for example, such as that disclosed in U.S. Pat. No. 4,271,486 assigned to the assignee of the present invention. These maximum (M) and minimum (m) values are actually stored as the acquired data points along the waveform in place of the instantaneous values actually occuring at those points. Hence, the maximum and minimum data are stored in a digital memory interleaved (M, m, M, m . . . ), i.e., the maximum and minimum data are respectively stored in even and odd address locations in the memory. For the envelope display, in a first cycle, the data stored in the even and odd addresses of the memory are read so that the read-out data representing the maximum and minimum signal values are alternately displayed on a display device by use of an associated vector generator. The data stored in only the even addresses of the memory are read so that the maximum values are displayed on the display device in a second cycle, and the data stored in only the odd addresses are read for displaying the minimum values in a third cycle. The three resulting displays are superposed on the display device, which appears to an observer as a single display of the filled-in envelope of the complex waveform.
Since this conventional system requires three cycles through the memory to display the envelope waveform, it needs complex hardware, software and time expense. In addition, the conventional system also requires a complex averaging filter to draw semistraight vectors between the maximum and minimum points.